A quick check on Google reveals there are ~200M French language web pages using .fr, 120M .com, and 25M .net. Looks like the French prefer using .fr but not by much.
Definitely if you target the French market.
But .fr is heavily regulated. As said above the rules will be relaxed next month (local address would be enough). Because it's so regulated there are only about 400000 or so .fr names active today. The French often use .com instead of .fr (cheaper and less hassle).
The registry is dropping the cost down to 6.50 euros a name come June 1st, and regulations will greatly relax on the 20th. Should be interesting to see what happens.